1. Field of the Invention
This invention relates to sound translating devices and more particularly to a method of and means for enhancing the fidelity of reproduction, in the bass acoustic range, of audio loudspeaker systems of the "acoustic suspension" type.
2. Description of the Prior Art
Audio loudspeaker systems that operate on acoustic suspension or bass-reflex principles generally employ a single electrically driven bass driver. This has required for the production of adequate low frequency sound output a "woofer", or bass driver diaphragm of relatively large size compared to the size of the enclosure, that is, cabinet or box, in which the diaphragm is housed.
One of the problems inherent in any loudspeaker system using relatively large woofer diaphragms in relatively small boxes is that the low frequency response of the system is impaired. In order to obtain response to lower frequencies, the total woofer diaphragm area has to be made smaller or the box larger, or both. Small woofer diaphragms are undesirable, however, because the available sound power output is reduced. Big boxes are undesirable because of their large physical size.
An audio loudspeaker system that operates on a so-called "active bass-reflex" principle is disclosed in U.S. Pat. No. 3,984,635 granted on Oct. 5, 1976 to the present inventor and Mioljub R. Nestorovic as copatentees, which patent hereinafter is referred to as the "Nestorovic" patent. This system bears a superficial similarity to that of the present invention, but as explained herein, operates on entirely different principles.
In the system of the Nestorovic patent, two drivers are employed, one of which is active at all frequencies of the low frequency range, and the other of which operates both electrically and acoustically in parallel with the first driver at the upper end of the range but behaves similarly to a passive sound radiator at the lower end of that range. Consequently, at low frequencies, near zero, the woofer diaphragms operate out of phase by substantially 180.degree.. As the frequency is increased, the diaphragms are gradually driven more in phase, the phase difference approaching 0.degree. at the upper frequency limit of the woofers.
While an improvement on loudspeaker systems operating on the "bass-reflex" principle, this system of the Nestorovic patent leaves something to be desired in that there is an undesirable out of phase diaphragm motion at subsonic frequencies and excessive sound output at the upper bass frequencies. Additionally, the system exhibits an uneven impedance characteristic that causes an undesirably varying load to the amplifier driving the speakers.